Generally, an electric vehicle is loaded with a motor as a drive unit and a battery unit serving as a power supply, both of which are mounted on a body of the electric vehicle. A travel distance of the electric vehicle depends upon performance and capacity of the battery unit. The battery unit includes a plurality of rechargeable battery modules. However, not only repeated charges and discharges but also temperatures tend to age battery modules relatively quickly. When a plurality of battery modules are provided, they will be differently aged depending upon their installed locations. Therefore, it is very difficult to judge which and how battery modules are aged. Japanese Patent Laid-Open Publication Hei 7-311248 discloses a method in which voltage measuring wires are connected to battery terminals during maintenance, and aged batteries are identified based on voltage variations obtained by the measuring wires.